1. Field of the Invention
The invention relates to a diagnostic apparatus for an internal combustion engine and, more particularly, to a diagnostic apparatus for a multicylinder internal combustion engine which is capable of executing an individual-cylinder EGR (Exhaust Gas Recirculation) in which EGR gas is distributed and supplied individually to the cylinders.
2. Description of Related Art
In a multicylinder internal combustion engine that performs the individual-cylinder EGR as described above, a plurality of branch EGR passageways are connected one-to-one to a plurality of branch intake passageways that are provided separately for each cylinder. It can happen that in the branch EGR passageway of one of the cylinders, deposit or the like collects, and blocks (or clogs) the passageway. If such a blocking abnormality occurs, amounts of EGR gas vary among the cylinders, resulting in air/fuel ratios varying among the cylinders, which, in some cases, can deteriorate exhaust emissions.
It is desirable that such an air/fuel ratio variation as to deteriorate exhaust emissions as described above be detected as an abnormality. Particularly, in the case of the internal combustion engines used in motor vehicles, in order to prevent a vehicle from traveling with deteriorated exhaust emissions, it has been demanded that an inter-cylinder air/fuel ratio variation abnormality of the engine be detected in a vehicle-mounted state (so-called on-board diagnostics (OBD)).
For example, Japanese Patent Application Publication No. 2010-156295 (JP 2010-156295 A) discloses as an example of the aforementioned countermeasure. That is, when the EGR apparatus is operated, the incidence (rate of occurrence) of knock is found separately for each cylinder by a knock sensor, and ratios of the incidence of knock among the cylinders are calculated. Then, differences between the incidences of knock of the individual cylinders and pre-stored initial incidences of knock of the cylinders are found to diagnose whether there is a deteriorated state of EGR distribution.
In the case where an inter-cylinder air/fuel ratio variation abnormality has occurred, conceivable causes of the abnormality include blockage of the branch EGR passageway of the abnormal cylinder, that is, an EGR system abnormality, a failed injector or the like, that is, a fuel system abnormality, and blockage of a branch intake passageway, that is, an air system abnormality. As for the diagnosis, it is more convenient if the cause of the abnormality or the location of the abnormality can be pinpointed. This contributes to prompt repair afterwards, or the like.